This invention relates generally to vapor deposition apparatus and methods for producing doped semi-conductors and particularly to the deposition of silicon dioxide films on silicon wafers. Still more particularly, this invention relates to rapid, relatively low temperature vapor deposition of silicon dioxide films on silicon wafers while eliminating unwanted deposition of the silicon dioxide film in the deposition chamber.
There is considerable interest in low-temperature techniques for depositing silicon dioxide films on silicon substrates to reduce dopant redistribution, wafer warpage, defect generation and to provide an insulator which requires no high-temperature steps for double level metallization. Deposition at low temperature also permits the use of layered photoresist-silicon dioxide-photoresist structures for high resolution lithography. Atmospheric chemical vapor deposition and low temperature low pressure chemical vapor deposition techniques, while reducing process temperatures, are deficient in uniformity, purity and film stability.
Plasma enhanced chemical vapor deposition techniques have made low temperature deposition temperature possible with improved physical characteristics, but plasma techniques are not always non-destructive, especially for radiation sensitive metal-oxide-semiconductor (MOS) devices. In plasma assisted chemical vapor deposition the substrate is bombarded with energetic neutral particles, charged particles and vacuum ultraviolet (VUV) photons, all of which contribute to chemical and physical damage to the substrate, the interface and the growing film. Another disadvantage of plasma deposited films is that the plasma potential is always more positive than the walls of the deposition chamber. Therefore, ions are accelerated by sheaths at the walls, thereby enhancing impurity sputtering and flaking, both of which degrade film quality. Plasma process parameters such as radio frequency power, radio frequency, gas flow, electrode spacing, total pressure, and substrate temperature are so interrelated that it is impossible to characterize and control defects due to a single parameter.
Because of the difficulties associated with atmospheric chemical vapor deposition, low temperature chemical vapor deposition and plasma enhanced chemical vapor deposition techniques, interest in photochemically deposited insulating films in which the reaction energy is selectively provided by photons has increased considerably. Previous workers have used both mercury photosensitized reactions and direct photolytic reactions to deposit silicon dioxide at low temperatures. Mercury lamps provide incoherent ultraviolet strong photons and vacuum ultraviolet weak photons to liberate atomic oxygen from molecular donor molecules by photodissociation. The use of mercury lamps causes the entire illuminated volume of gas to react to form products, Unwanted deposition and loss of reactants on reactor walls may be considerable and deposition rates are low. The best mercury sensitized deposition rate is just under the 200 A/min. The limitation of deposition rate is attributed to loss of atomic oxygen by recombination on surfaces of the reactor vessel.
High deposition rate is of concern in economical production processes and can ultimately determine film purity, given the background pressure of impurities and the desired film thickness. Therefore, there is a need in the art for new methods of film deposition, which improve the characteristics of inter-layer dielectrics, such as step coverage, uniformity, film integrity, speed of deposition and elimination of unwanted deposition and loss of reactants on reactor walls.